Question

2. Explain: - (i) Rusting of Iron (ii) Corrosion of copper (ii) Corrosion of silver


With the help of suitable chemical reaction.
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Answer 1

Answer:

The Statue of Liberty is a landmark every American recognizes. The Statue of Liberty is easily identified by its height, stance, and unique blue-green color (Figure 1). When this statue was first delivered from France, its appearance was not green. It was brown, the color of its copper “skin.” So how did the Statue of Liberty change colors? The change in appearance was a direct result of corrosion. The copper that is the primary component of the statue slowly underwent oxidation from the air. The oxidation-reduction reactions of copper metal in the environment occur in several steps. Copper metal is oxidized to copper(I) oxide (Cu2O), which is red, and then to copper(II) oxide, which is black.

2Cu(s) + \displaystyle \frac{1}{2}

2

1

O2(g) ⟶ Cu2O(s) (red)

Cu2O(s) + \displaystyle \frac{1}{2}

2

1

O2(g) ⟶ 2CuO(s) (black)

Coal, which was often high in sulfur, was burned extensively in the early part of the last century. As a result, sulfur trioxide, carbon dioxide, and water all reacted with the CuO

2CuO(s) + CO2(g) + H2O(l) ⟶ Cu2CO3(OH)2(s) (green)

3CuO(s) + 2CO2(g) + H2O(l) ⟶ Cu2(CO3)2(OH)2(s) (blue)

4CuO(s) + SO3(g) + 3H2O(l) ⟶ Cu4SO4(OH)6(s) (green)

These three compounds are responsible for the characteristic blue-green patina seen today. Fortunately, formation of the patina created a protective layer on the surface, preventing further corrosion of the copper skin. The formation of the protective layer is a form of passivation, which is discussed further in a later chapter.

Answer 2

Answer:

• Rusting is an oxidation reaction. The iron reacts with water and oxygen to form hydrated iron.
• Copper corrosion is the corrosion of materials made of copper or copper alloys. When exposed to the atmosphere, copper oxidizes, causing normally bright copper surfaces to tarnish.
• When silver is attacked by sulfur, the chemical reaction leaves the metal with a brown-black patina which does not result in much metal loss. The attack also results in black fissures and pits.